Intelligent isolation systems are used to control input disturbances or forces transmitted between relatively movable members such as a vehicle body (a sprung mass) and a support or vehicle wheel (an unsprung mass) in order to provide desired ride characteristics.
Semiactive isolation systems are a type of intelligent isolation system which require no external energy other than that needed to actuate valves, and power the sensors and controls, yet are capable of providing for rapid changes in the damping coefficient of a damper interconnecting the members so as to optimize the attenuation of forces between the members. Semiactive systems are distinguished from fully active systems which employ an external power source or force generator for supplying energy in a controlled manner to counteract vibrational forces. Fully active systems are disadvantageous in that they require a large auxiliary power source and are not fully responsive at high operating frequencies due to inadequacies of such equipment to respond rapidly to control signals. Fully active systems are also characterized by dangerous failure modes.
A limitation of semiactive systems is that they only are capable of creating a force opposing motion in a suspension system and cannot create a force in the direction of motion. Thus, the term "semiactive" refers to control systems which are only capable of removing energy from a suspension system. Semiactive systems are nonetheless capable of performance approaching that of a fully active system when operated pursuant to a suitable control policy, and in particular those control policies which emulate a hypothetical "skyhook" damper as described in Karnopp, D. C. et al., "Vibration Control Using Semiactive Force Generators," ASME Paper No. 73-DET-123 (June 1974), incorporated herein by reference. Semiactive dampers and control policies for them, are disclosed in Karnopp, U.S. Pat. No 3,807,678; Miller et al., U.S. Pat. Nos. 4,821,849, 4,838,392 and 4,898,264; Boone, U.S. Pat. No. 4,936,425; and Ivers, U.S. Pat. No. 4,887,699, all owned by the assignee of the present invention. The disclosures of the foregoing patents are incorporated herein by reference.
Dampers used in a semiactive system may be either of the "off/on" type, of the "orifice-setting" type, or of the "force-controlled" type. An "off/on" semiactive damper is switched, in accordance with the dictates of a suitable control policy, between alternative "on" and "off" damping states or conditions. In the "on" state, the damping coefficient of the damper is of a preselected, relatively high magnitude. The term damping coefficient as used herein is understood to mean the relationship of the damping force generated by the damper to the relative velocity across the damper, which relationship is not necessarily linear. In its "off" state, the damping coefficient of the damper is approximately zero or of a relatively low magnitude sufficiently greater than zero so as to discourage "wheel hop". An orifice-setting semiactive damper is also switched during operation between an "off" state, where the damping coefficient is approximately zero or of some relatively low magnitude, and an "on" state. However, when a orifice-setting semiactive damper is in its "on" state the damping coefficient thereof may be and normally is changed between a large (theoretically infinite) number of different magnitudes. The magnitude of the damping coefficient is typically determined by the diameter setting of the valve orifice in the damper.
A "force-controlled" damper, in theory, is capable of creating any desired dissipative force in the "on" state independent of the relative velocity across the damper. This is in contrast to the aforementioned "off/on" and "orifice-setting" dampers in which the damping force in the "on" state depends on the relative velocity across the damper. A force-controlled damper can either be realized by use of feedback control or by use of pressure control valves. In the "off" state the force-controlled damper will command the valve to the full-open position in which the damping coefficient is approximately zero or of some relatively low value.
A characteristic of semiactive isolation systems employing an above-described "skyhook" control policy or a derivative thereof is that they tend to increase the average rang of suspension deflection from the equilibrium position in order to provide a "smoother" ride. Although these systems offer significant performance advantages over other types of isolation systems, difficulties can occur when the suspension is subjected to large, abrupt input disturbances such as those encountered on rough terrain or upon the landing of an aircraft, for example. Undesirable transient responses in the form of excessive vehicle body motions or suspension excursions result in uncomfortable or damaging force inputs to a vehicle throughout the range of motion of the suspension and also when the suspension experiences abrupt impacts upon reaching its end of travel. This tendency is discussed in Miller, "Tuning Passive, Semiactive and Fully Active Suspension Systems," Proceedings of the 27th CDC of IEEE, Vol. 3, 1988 and in Ivers et al., "Experimental Comparison of Passive, On/Off Semiactive and Continuous Semiactive Suspensions" SAE Paper No. 892484, Dec. 7, 1989.
The incidence of undesirable transient responses could be reduced or even eliminated by use of a very stiff damper with a high damping coefficient. However, this would defeat the performance advantages of semiactive control by unnecessarily limiting the available range of suspension deflection for the given range of motion of the suspension and unacceptably degrade the isolation of the vehicle.